#3176 Web app: use wide windows and fit narrow screens

Descrição

The web app's views were capped at fixed widths (chat 860 px, monitor 1240 px), so on a wide window, or in a wide side panel, most of the space stayed empty. This makes the layout follow the window:

- **Wide windows:** the monitor grows up to 1920 px and its metric cards fill the row (`auto-fit`, 280 px minimum, so 1280 px keeps 4 columns); chat, composer and About grow to 1100 px while message text keeps a readable line length.
- **Narrow screens:** breakpoints at 900, 640, 560 and 480 px. The monitor row stacks, metrics drop to one column, the settings form becomes one column with full-width inputs, gauges scale instead of using fixed pixel sizes, long values (paths, model names) wrap, and touch targets are at least 40 px tall.

Only `serve/web/app.css` changes; no markup, script or token changes.

**Checked:** `document.scrollWidth <= innerWidth` on Chat, Monitor, About and the API monitor page at 360, 390, 768, 1280 and 1920 px; used live for a day on macOS (Metal build), Chrome, in a normal window and a split-view side panel.

One note for testing: the server sends the web files without cache headers, so browsers can keep the old stylesheet; a hard reload (Cmd/Ctrl+Shift+R) shows the change.